That's not true at all. Theories are based on facts, 100% true 100% of the time. The theory itself is merely the vehicle that explains this fact, there is a rigid definition of theory and what is applicable as such. The theory can only ever be an explanation.

For an analogy, if you are on a jury for a murder trial there is the simple fact that someone died. However the evidence forms the explanation as to how it happened, even with a recording of the murder, the jury still forms a theory.

Also, there are things that are 100% knowable, these are delegated as laws because they apply everywhere in the universe, such as the Second Law of thermodynamics, which in an abridged version dictates all energy from order to disorder.